From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755549Ab0JONfI (ORCPT ); Fri, 15 Oct 2010 09:35:08 -0400 Received: from smtp-out.google.com ([216.239.44.51]:24309 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755275Ab0JONfG (ORCPT ); Fri, 15 Oct 2010 09:35:06 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=message-id:from:date:to:reply-to:cc:subject; b=BosyPz8583mDi7bz9oTDBE7bIAGmK7MqSlr3e/6XChnAlKSNJIRC7ea51QUuJi934/ 1pGjanf+3zO88VJ2Qrew== Message-ID: <4cb856dc.51edd80a.5ae0.38fb@mx.google.com> From: Stephane Eranian Date: Fri, 15 Oct 2010 15:26:01 +0200 To: linux-kernel@vger.kernel.org Reply-to: eranian@google.com Cc: peterz@infradead.org, mingo@elte.hu, paulus@samba.org, davem@davemloft.net, fweisbec@gmail.com, perfmon2-devel@lists.sf.net, eranian@gmail.com, eranian@google.com, robert.richter@amd.com Subject: [PATCH] perf_events: fix bogus context time tracking X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org You can only call update_context_time() when the context is active, i.e., the thread it is attached to is still running. However, perf_event_read() can be called even when the context is inactive, e.g., user read() the counters. The call to update_context_time() must be conditioned on the status of the context, otherwise, bogus time_enabled, time_running may be returned. Here is an example on AMD64. The task program is an example from libpfm4. The -p prints deltas every 1s. $ task -p -e cpu_clk_unhalted sleep 5 2,266,610 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982) 0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982) 0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982) 0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982) 0 cpu_clk_unhalted (0.00% scaling, ena=2,158,982, run=2,158,982) 5,242,358,071 cpu_clk_unhalted (99.95% scaling, ena=5,000,359,984, run=2,319,270) Whereas if you don't read deltas, e.g., no call to perf_event_read() until the process terminates: $ task -e cpu_clk_unhalted sleep 5 2,497,783 cpu_clk_unhalted (0.00% scaling, ena=2,376,899, run=2,376,899) Notice that time_enable, time_running are bogus in the first example causing bogus scaling. This patch fixes the problem, by conditionally calling update_context_time() in perf_event_read(). Signed-off-by: Stephane Eranian --- diff --git a/kernel/perf_event.c b/kernel/perf_event.c index b6fd9ec..4dae345 100644 --- a/kernel/perf_event.c +++ b/kernel/perf_event.c @@ -1775,7 +1775,13 @@ static u64 perf_event_read(struct perf_event *event) unsigned long flags; raw_spin_lock_irqsave(&ctx->lock, flags); - update_context_time(ctx); + /* + * may read while context is not active + * (e.g., thread is blocked), in that case + * we cannot update context time + */ + if (ctx->is_active) + update_context_time(ctx); update_event_times(event); raw_spin_unlock_irqrestore(&ctx->lock, flags); }